Overwintered onions in the Willamette Valley are seeded in early September for harvest the following spring. Strong growth in the spring is essential for producing high value jumbo bulbs. However, air and soil temperatures in the spring are less than optimal, possibly limiting response to fertilizers.
Phosphorus availability is limited on cold soils; thus overwintered onions might respond to applications of P fertilizers or lime, which increased P availability. Onions also have a high S requirement, but overwinter onion response to a fertilizer S source had not been previously studied in the Willamette Valley.
This trial was the third in a series commencing in 1982/83. Onion yields increased markedly with liming in 1982/83 and 1983/84, both because of increased stands and to increased bulb size. Onion yield did not vary significantly with a broadcast application of superphosphate in the spring in either year, except for a very small increase in mean bulb size. Yields increased slightly with spring-applied gypsum (CaSO4) in 1982/83, indicating a possible S response.
Another earlier trial resulted in slightly higher yields with ammonium sulfate (21-0-0-24) rather than ammonium nitrate (34-0-0) as N source. It was not determined whether this was a response to sulfur in the ammonium sulfate or indicated an advantage to a 100 percent ammonium-N source.
The purpose of the 1984/85 trial was to further evaluate the response of overwintered onions to lime, gypsum, an at-planting banded application of superphosphate, and various sources of N.
Methods
Agricultural limestone (95% CaCO3 equivalent) at 0, 2, 4, and 6 tons/acre was applied in 1979 to 2,300-square-foot plots of Willamette silt loam with four replications of each lime rate in randomized block design. Soil pH in 1982, at the start of this series of experiments, averaged 5.5, 6.0, 6.2, and 6.6, respectively. Raised beds (8 inches high, 5 feet wide) were formed in early September 1984, following a broadcast application of 700 pounds/acre of 10-20-10 fertilizer, and were seeded with three rows/bed of Sweet Winter (ARCO Seed Co.) onion on September 12. The lime plots were split by application of concentrated superphosphate at 0 or 90 pounds/acre in a band 2 inches to the side and 2 inches beneath the seed row.
Propachlor herbicide was applied at 4 pounds/acre at planting, and again on October 17, December 6, February 22, and May 7. Metalaxyl fungicide was applied at 8 ounces/acre on October 17 and April 4. Chloroxuron herbicide was applied on February 22. Plots were also hand-hoed twice to control grasses, vetch, and late-germinating groundsel. On February 18, gypsum was broadcast on the appropriate plots at 150 pounds/acre. Ammonium nitrate was applied at 50 pounds N/acre to all plots on February 18, March 25, and May 7. Leaf samples were collected for tissue analysis on May 1. Plants were topped and harvested on July 16. Soil samples were collected for pH determination following harvest.
For the N source experiment, methods were similar to the above. Seeding was on September 11. On February 18, the appropriate plots were treated with a broadcast application of 50 pounds N/acre as calcium nitrate (15.5-0-0), ammonium nitrate, ammonium sulfate, urea (46-0-0), ammonium chloride (38-0-0), or ammonium nitrate plus 150 pounds gypsum/acre. The N treatments were re-applied on March 25 and May 7. Leaf samples were collected for tissue analysis on May 1. Plants were topped and harvested on July 16.
Results
As in the preceding years, onion stands increased with increasing soil pH (lime application), and this is reflected in the greater number of bulbs harvested at the higher rates of lime (Table 1). In the previous trials, most of the stand increase with lime occurred with application of only 2 tons/acre. In 1984/85, however, the stand was significantly higher at the 4 tons/acre than at the 2 tons/acre rate. This may have been because of the general decline in soil pH of the lime-treated soil over the intervening years (Table 2). The sharp decline in soil pH between 1982 and 1985 may be partially explained by the heavy applications of acid-forming N fertilizers during this time. Another contributing factor is that the sampling in 1982 occurred in the spring with saturated, well-leached soil. The 1985 sampling was on dry soil with high residual fertilizer content, which would tend to produce lower readings. Overall stands and yields were lower in 1984/85 than in 1982/83, primarily because of reduced seeding rather than reduced emergence.
Liming also greatly increased the mean bulb size and percentage of large (grade No. 1) bulbs (Table 1). The combination of increased stands and greater bulb size contributed to a nearly 8-fold increase in total yield between the lowest and highest rates of lime. This confirms the 4 to 9-fold yield increases with lime in the previous years.
Lime had no effect on leaf tissue concentration of K, Ca, and Cu, but reduced concentrations of N, Mg, Zn, and Mn and increased leaf S concentration (Table 3). The reduced Mg content may be a dilution effect of increased leaf growth or may reflect competition for uptake between Mg and Ca. The large reduction in Mn concentration on limed soil and the high level in tissue grown on unlimed soil indicate that Mn toxicity may play a role in poor onion growth at low pH.
Banding P at planting had no effect on plant stands (Table 1), but increased the mean bulb weight, reflected in an increase in yield of grade No. 1 bulbs and the percentage of No. 1 bulbs. Total yield also tended to be increased with banded P, but the difference was not significant. This was in contrast to the previous trials, in which the response to broadcast P was very small and usually not statistically significant. Banded P reduced leaf tissue concentrations of N, S, Mn and Cu, but the differences were small (Table 3).
Gypsum application slightly increased the percentage of No. 1 bulbs and mean bulb weight, but the increases were not significant. The number of bulbs harvested and total yield tended to decrease with gypsum. These results are in contrast to 1982/83, when all components of No. 1 and total yield were increased with gypsum application and the gypsum response was greatest at higher soil pH. Gypsum application increased leaf tissue N and S concentrations.
There were no significant 2- or 3-way interactions of lime, gypsum, and P affecting any yield component or leaf tissue elemental concentration, thus only main effects are reported in the tables.
Table 1. Main effects of lime, banded P, and gypusm on yield components of overwintered onions, 1984-1985 Stand Total bulbs No. 1 bulbs Total No. 1 Mean wt. Mean wt. Percent seedlings/ harvested/ harvested/ yield yield All bulbs No. 1 bulbs No. 1s plot plot plot Lime (tons/acre) --------------No./24 ft----------- --tons/acre- --------ounces-------- % 0 22 18 0.5 1.6 0.1 1.9 10.0 2 2 41 35 3.8 5.6 1.4 4.3 9.8 11 4 62 55 10.0 11.1 3.8 5.9 10.7 19 6 63 57 13.2 12.3 5.2 6.3 10.7 26 LSD (0.05) 9 15 5.1 2.9 0.8 0.6 0.5 9 + P 46 42 8.4 8.4 3.2 5.0 10.7 18 - P 48 41 5.3 6.9 1.9 4.2 10.1 11 NSZ NS ** * ** * NS ** + Gypsum 47Y 40 6.9 7.5 2.8 4.8 10.5 15 - Gypsum 47 43 6.8 7.9 2.5 4.5 10.3 14 NS NS NS NS NS NS NS NS Z*, *, NS: significant differences among means at 1% and 5% levels, and non-significant, respectively. YStand recorded before gypsum was applied. Table 2. Effect of liming on soil pH for samples taken in 1982 and 1985 Lime rate (tons/acre) Soil pH, 1982 Soil pH, 1985 0 5.5 4.5 2 6.0 4.7 4 6.2 5.0 6 6.6 5.2 Table 3. Effects of lime, banded P, and gypsum on onion leaf tissue elemental concentrations Treatment N P K Ca Mg S Zn Mn Cu Lime (tons/acre) ----------------%------------------- ------ppm----- 0 3.8 0.16 2.04 0.90 0.169 0.22 24 323 4.0 2 3.6 0.17 2.06 0.81 0.138 0.26 20 126 4.0 4 3.6 0.18 2.27 0.85 0.134 0.28 18 79 4.2 6 3.3 0.15 2.16 0.81 0.127 0.27 17 61 3.9 LSD(0.05) 0.4 NS NS NS 0.017 0.03 5 87 NS + P 3.4 0.16 2.09 0.83 0.141 0.24 20 140 3.7 - P 3.6 0.16 2.17 0.85 0.143 0.27 20 154 4.4 *Z NS NS NS NS * NS * * + Gypsum 3.6 0.16 2.15 0.85 0.142 0.30 20 145 4.1 - Gypsum 3.4 0.17 2.12 0.83 0.142 0.22 20 149 3.9 * NS NS NS NS ** NS NS NS Z**, *, NS: significant differences at 1% and 5% levels, and no significant differences, respectively.
In the N source trial, the number of bulbs harvested per plot varied significantly with treatment, but the stands may have varied before the treatments were applied (Table 4). Total yield varied with N source, but the differences were not directly proportional to stand differences. Mean bulb weight and percent No. 1 bulbs were greatest with ammonium sulfate, in spite of a greater than average stand. They were lowest with ammonium chloride, in spite of a low stand. The high percentage of No. 1 bulbs and high mean bulb weight with ammonium sulfate confirms the efficacy of this fertilizer observed in 1982/83. Among the other fertilizers, there was no clear advantage for ammonium-N over nitrate-N sources. Adding the sulfur source, gypsum, to ammonium nitrate did not improve yields. Although the amount of S provided by the gypsum (36 pounds/acre) was much lower than that provided by the three ammonium sulfate applications (171 pounds/acre), it should have been sufficient to cause a yield response if S were deficient in the soil. Gypsum at this rate also did not increase yields in a parallel study of the effects of lime, P, and gypsum on overwintered onions in 1984/85. However, in a study of the effects of lime and gypsum on spring-seeded onions in 1985, this rate of gypsum increased yields significantly.
Source of N had no effect on leaf tissue concentrations of N, P, K, Ca, Mg, Zn, and Cu (Table 5). Leaf Mn concentration was highest with the acid-forming ammonium chloride and ammonium sulfate. These levels of Mn are all in the normal range and should not have affected yield. Leaf S concentration was highest with ammonium sulfate or with ammonium nitrate plus gypsum, indicating plant availability and uptake of the fertilizer sulfate. Although leaf S levels were low with the other N sources, there was no correlations between leaf S levels and yield, because of the low yields with ammonium nitrate plus gypsum.
While confirming that ammonium sulfate is a good spring N source for overwintered onions, this trial provided no new information on the relative importance of the ammonium or sulfate ions in providing the yield response. The increase in leaf Mn with ammonium sulfate, and tendency toward increased leaf Zn content with this fertilizer, may indicate that ammonium sulfate, through its acidifying effect on the soil, is increasing availability of micronutrients which were limited in availability on this well-limed soil.
Table 4. Effect of N source on yield of overwintered onions, 1985 Total bulbs No. 1 bulbs Total No. 1 Mean wt. Mean wt. Percent harvested/plot harvested/plot yield yield All bulbs No. 2 bulbs No. 1s --------No./24 feet----------- ---tons/acre-- -------ounces---------- Amm. nit. 54 12 10.9 4.6 5.9 10.4 23 Amm. sul. 68 19 15.0 6.7 6.5 10.0 28 Cal. nit. 59 15 12.2 5.3 6.2 10.4 26 Amm. chl. 55 3 7.4 1.1 3.9 9.7 6 Urea 74 15 12.8 4.8 5.1 9.5 19 Amm. nit.+gypsum 56 11 11.1 3.8 5.8 10.0 20 LDS (0.05) 10 3 4.0 2.8 1.4 NS 14 Table 5. Effect of N source on onion leaf tissue elemental concentrations, 1985 N Source N P K Ca Mg S Zn Mn Cu ----------------%---------------- -----ppm------ Amm. nitrate 3.9 0.20 2.34 0.82 0.13 0.22 21 47 4.6 Amm. sulfate 3.5 0.19 2.25 0.80 0.12 0.37 21 61 4.3 Cal. nitrate 4.0 0.22 2.39 0.82 0.13 0.21 19 47 4.1 Amm. chloride 3.3 0.20 2.23 0.82 0.12 0.18 20 86 4.1 Urea 3.4 0.19 2.17 0.88 0.12 0.22 18 51 3.9 Amm. nit.+gypsum 3.7 0.19 2.15 0.76 0.12 0.34 18 41 4.3 LSD(0.05) NS NS NS NS NS 0.05 NS 8 NS